This invention generally relates to two-dimensional distributed feedback (DFB) lasers. At least some embodiments of the invention relate to surface-emitting photonic crystal distributed feedback lasers and self-pumping self-focusing mechanisms.
An ideal semiconductor laser that would emit high power into a single spectral mode with diffraction-limited output profile is of great interest for a number of commercial and military applications, including spectroscopy, infrared countermeasures, wavelength multiplexing in telecommunications, and diode pump arrays. However, known laser configurations have been unable to provide the desired single-mode characteristics without sacrificing power output, and vice versa. For example, the distributed-feedback (DFB) configuration with its one-dimensional (1D) diffraction grating parallel to the laser facets routinely yields high spectral purity as long as the waveguide is narrow enough with typical widths of 2-5 microns for lasers emitting at 0.8-1.55 microns wavelengths) to considerably lower the gain of higher-order lateral modes with respect to the fundamental mode and thereby prevent their excitation. However, scaling up the stripe width for increased power leads to a loss of phase coherence across the DFB laser stripe, primarily owing to the self-modulation of the refractive index in the active region by non-uniformly distributed carriers. The output then has a broadened spectral profile, ultimately limited by the width of the gain spectrum, and a rapidly diverging, often double-lobed, far-field pattern.
An ideal semiconductor laser would produce a diffraction-limited circular output beam with high optical power and a single spectral mode. In broad-area distributed-feedback lasers lateral coherence is established by defining a grating in the epitaxial device structure. In spite of the great promise of edge-emitting Photonic-crystal distributed feedback (PCDFB) lasers, drawbacks include severe ellipticity of the output beam having a fast angular divergence along the growth axis and a slow divergence along the laser stripe. In prior SE PCDFB lasers, the use of electrical pumping obscured identification of the lasing model(s). The impact of PCDFB grating structure choice on device performance, and the relation to one-dimensional SE DFB lasers was not understood. The absence of a general theoretical description of SE PCDFB lasers precluded any systematic analyses of the limits to their single-mode operation, output power, and brightness.
The basic concept of employing 2D DFB gratings was proposed by Wang and Sheem (U.S. Pat. No. 3,970,959). However, there was no guidance provided concerning the proper choice of the device parameters or the 2D lattice structure.
Other prior approaches considered only two superimposed 1D gratings (rather than actual 2D gratings), in which only two diffraction processes are possible. Furthermore, realistic device geometries were not considered, and the critical role played by the linewidth enhancement factor (LEF) was not considered.
At least some embodiments of the invention relate to surface-emitting photonic crystal distributed feedback lasers and self-pumping self-focusing mechanisms.
According to one embodiment, a surface-emitting photonic crystal distributed feedback laser apparatus configured to output an optical beam of light is described. The apparatus includes a laser waveguide bounded by a top and bottom optical claddings, an active region configured to produce optical gain upon receiving optical or electrical pumping to inject electrons and holes into the active gain region, a periodic two-dimensional grating having an order higher than the fundamental, the grating configured to induce modulation of a modal refractive index and a lateral pumped gain area contained within an area covered by the grating and configured to produce gain in one or more lasing modes and the modal index of the one or more lasing modes is modulated by the periodic two-dimensional grating, the lateral pumped gain area having a substantially circular shape of diameter D, and wherein the pumped gain area is enclosed by an unpumped region contained within the area covered by the grating but not receiving the optical or electrical pumping.
According to another embodiment of the invention, a method of producing a diffraction-limited beam using a surface emitting photonic crystal distributed feedback laser. The method comprises providing a waveguide in a laser cavity bounded by top and bottom optical claddings; configuring an active region to produce optical gain upon receiving optical or electrical pumping; providing a periodic two-dimensional grating having an order higher than a fundamental, the grating configured to modulate a modal refractive index; confining a lateral pumped gain area to within an area covered by the grating, the gain area configured to produce gain in at least one lasing mode having a modal index modulated by the periodic two-dimensional grating, the lateral pumped gain area having a substantially circular shape of diameter D; and enclosing the gain area by an unpumped region contained within the area covered by the grating but not receiving the optical or electrical pumping.
According to an additional embodiment of the invention, a method of producing a diffraction-limited beam using a surface emitting photonic crystal DFB laser, comprising a) selecting one or more parameters for a two-dimensional grating to operate at a predetermined wavelength; b) calculating coupling coefficients xcexa1, xcexa2, and xcexa3 using at least the grating parameters; c) calculating output power and beam quality of the laser; d) determining if the output power corresponds to a desired quantum efficiency; and e) determining if the beam quality is in a desired range relative to the diffraction limit.
Other aspects of the invention are disclosed herein as is apparent following description and figures.